Re: [PATCH] mm/rmap: fix the handling of device private page in try_to_unmap_one()

[Michal Hocko <mhocko@kernel.org>]

On Sun 22-03-20 21:57:07, Pingfan Liu wrote:
> For zone_device, migration can only happen on is_device_private_page(page).
> Correct the logic in try_to_unmap_one().

Maybe it is just me lacking knowledge in the zone_device ZOO. But
this really deserves a much more detailed explanation IMHO. It seems
a5430dda8a3a ("mm/migrate: support un-addressable ZONE_DEVICE page in
migration") deliberately made the decision to allow unmapping these
pages? Is the check just wrong, inncomplete? Why?

What is the real user visible problem here?
 
> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> Cc: Jérôme Glisse <jglisse@redhat.com>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> To: linux-mm@kvack.org
> ---
>  mm/rmap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index b838647..ffadf3e 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1380,7 +1380,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> 
>  	if (IS_ENABLED(CONFIG_MIGRATION) && (flags & TTU_MIGRATION) &&
>  	    is_zone_device_page(page) && !is_device_private_page(page))
> -		return true;
> +		return false;
> 
>  	if (flags & TTU_SPLIT_HUGE_PMD) {
>  		split_huge_pmd_address(vma, address,
> @@ -1487,7 +1487,7 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> 
>  		if (IS_ENABLED(CONFIG_MIGRATION) &&
>  		    (flags & TTU_MIGRATION) &&
> -		    is_zone_device_page(page)) {
> +		    is_device_private_page(page)) {
>  			swp_entry_t entry;
>  			pte_t swp_pte;
> 
> --
> 2.7.5

-- 
Michal Hocko
SUSE Labs